Predictive Supply Temperature Optimization of District Heating Networks Using Delay Distributions

Abstract

Fluctuating power production in combined heat and power (CHP) plants may cause unwanted disturbances in district heating (DH) systems, which leads to the situation that the best efficiency in CHP production is not achieved. DH -systems are often automated, however, supply temperature is still primarily chosen manually by the operator. This is because of the uncertain heat demand in near future and uncertain behaviour of delay from heat supplier to consumers, which make the temperature scheduling problematic.<br/><br/>In this work, future heat demand and return water temperature are predicted based on outdoor temperature forecast and process data history using neural network estimators. Consumers in network are presumed to be similar, but their distances from production site vary thus creating a distribution function of range. Delay is modelled as a distribution function based on the distances between heat consumers and the suppliers, which weights the supply temperatures from last few hours calculating the average supply temperature received by the consumers. The derived function models how the temperatures develop along the network, finally covering the entire network.<br/><br/>A brute force optimizer was developed to minimize both pumping costs and heat losses as well as to smooth temperature gradient originated thermal stresses. System delays are fixed during an optimization cycle, and after each cycle the delays are updated according to new system flowing rates and the optimization is recalculated. The resulting supply temperature curve is a discrete curve that cuts the heat load peaks by charging and discharging the energy content of the District heating network (DHN). Optimization keeps the supply water temperature and flow rates in control and stabilizes the network smoothly and efficiently after disturbances. Optimization is demonstrated by using case data of one year from a district heating system in Finland.